This invention concerns a hydraulic dual-circuit external power brake valve of tandem design.
Such dual-circuit external power brake valves have two control pistons independent of one another, each of which provides for one of the two brake circuits. The two control pistons are located in series in the dual-circuit external power brake valve.
In dual-circuit external power brake valves with control pistons arranged in series, the regulating spring or main spring must be designed so that an equilibrium is reached between the pressure forces on the faces of the control pistons plus the spring force of the restoring spring.
In the hydraulic dual-circuit external power brake valves known up to now, this situation results in the following drawbacks under high brake pressures: the structural space necessary for the main spring or regulating spring is relatively large. In order to reduce the structural space, the spring constant of the regulating spring must be chosen to be disproportionately high. This impairs the apportioning capability of the entire brake system. Furthermore, the regulating reactions of the control pistons can be felt in the foot of the driver. To reduce the restoring forces of the control pistons, the diameter of the control pistons must be reduced, whereby acceptable spring dimensions can be achieved. However, this presents manufacturing difficulties in the production of the case of the dual-circuit external power brake valve.